Why is it that diamonds are the best gem to buy?

The are the best to buy if your the one selling them . The resale value is not very good at all, so they are not a good investment.

The supply is artificially limited , so the retail prices stay high.

There is a huge amount of advertising to convince women that diamonds are what they should get if a guy loves them and therefore affects what men actually buy.
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I was not aware that they were...

In fact, I think they are responsible for wars, death, violence and crime....

I think they are nasty little rocks that good people would do well to avoid.

Because diamonds never depriciate in value

They tend to not only to hold their value but to appreciate in value over the years.

Well, diamonds make a girls mouth drop to the floor..they are beautiful and they look good on girls. They are the strongest type of rock on this planet and the only thing that can split a diamond in half is another diamond. Overall,there value does not drop and the quality is terrific!

There is a large supply of diamonds on this planet, however, most of it is controlled by the De Beers monopoly, who make sure only little portion of it sees the market, so their value would not drop.

They are not inherently the "best" gem to buy, it's just where their market is right now. There is no guarantee it will stay this way, as this is a social state, nothing more.

because diamonds are the strongest minerals